A horse’s visual perception is considerably different than ours in regard to colours, adaptation to light, visual fields and acuity. Sight is vital to survival in the wild, but it also has a considerable impact on the equine athlete. Being able to judge the distance and height of a jump, as well as evaluating terrain, are crucial elements that can affect the performance of the sport horse.
If you’ve been struggling with behavioural issues such as head-shaking or spookiness, it could possibly be a sign that something is amiss with your horse’s vision. Corpora nigra cysts could be the culprit.
What are corpora nigra cysts in horses?
While looking into a horse’s eye in bright, sunny conditions, you may notice the tiny brown oddly-shaped formations along the top of the horse’s pupil. These are the corpora nigra. Attached to the iris (the coloured part of the eye), it’s believed that the main function of this unique structure is to shade the pupil from excessive glare. In some cases, this structure may become cystic, causing the corpora nigra to be larger than normal or in other instances, causing brown-coloured enlargements to form elsewhere along the edge of the pupil. These cysts look like dark bubbles and can best be observed by shining a light into the horse’s eye.
While a fairly common condition and generally benign – in many cases not requiring treatment or intervention – the cysts can affect vision. The eye actually looks normal to anyone observing the horse, as the cysts only become visible under bright light. For this reason, many horses go undiagnosed until a routine eye exam is performed by a veterinarian. If the cysts are large enough to obstruct the pupil, the horse may experience blind spots in its vision. This hindrance to vision can vary according to light conditions, depending on the size of the pupil (whether fully constricted or fully open).
The condition does not cause any pain or discomfort, therefore horses do not present the typical symptoms of eye pain including a semi-closed or closed eyelid and runny eye which are often associated with other conditions such as uveitis. Typical symptoms instead include behavioural and performance issues such as headshaking, spooking, or refusing when jumping. Depending on the location and size of the cyst, some horses only display behavioural issues under certain light conditions such as full sun when the pupil is the most constricted and the cyst will be more obstructive to vision, but not if the sky is overcast. However, many horses do not present any symptoms whatsoever.
Treatment for corpora nigra cysts
Following an eye exam, your veterinarian may refer your horse to an equine ophthalmologist to confirm the diagnosis and to discuss treatment options. Other conditions such as equine iris melanomas (tumours) will also need to be ruled out, as they can resemble cysts but are much more serious.
Once the condition is properly diagnosed, a simple procedure using lasers can be performed in order to deflate the cysts that are obstructing vision. With vision restored and blind spots eliminated, the behavioural issues they were causing should also be resolved. The procedure is generally very successful, but cysts can occasionally return, so regular eye exams are key.